As shown in FIG. 3, such a ribbon microphone is provided with an acoustic-electric converter (i.e. microphone unit) 1. The converter 1 has a metallic foil such as an aluminum foil in the form of a strip of several micrometers thick as a diaphragm 10. The foil is placed in a parallel magnetic field formed by a pair of permanent magnets 30, 30 facing each other with a predetermined space therebetween.
Attachment electrodes 20, 20, which include a pair of support electrode plates 20a and 20b, are attached to opposite ends 10a and 10b of the diaphragm 10. The attachment electrodes 20, 20 are connected to a step-up transformer (not shown) on the primary winding side of the transformer.
The step-up transformer is connected to a 3-pole (or 3-pin) output connector on the secondary winding side of the transformer. When the microphone is used, a power plug of a phantom power supply is connected to the output connector. The ribbon microphone is thus operable with the phantom power supply.
Since such a ribbon microphone is bi-directional and mass controlled, the resonance frequency can be significantly lowered and sounds can be collected in a lower tone range.
A problem with a ribbon microphone is that, on impact against the microphone, an inertial force of the diaphragm (sometimes referred to as “ribbon” hereinafter) 10 stretches the ribbon foil, leaving it in an elongated state by plastic deformation. When a ribbon plastic-deformed in this way contacts a magnetic pole or a nearby component, the performance may significantly be degraded.
For this reason, it is a common practice to provide protections against shocks during transportation such as cushioning materials attached to the inside and/or outside of a box containing a microphone so as to avoid a direct impact on the microphone.
Besides shocks during transportation, however, other shocks on impact against a microphone include a drop impact experienced, for example, when the microphone is accidentally dropped while being handled on a microphone stand for attachment. It is therefore necessary to protect a ribbon against shocks when the microphone is not in use (i.e. when the phantom power supply is not connected) during other times than transportation.
Thus, the applicant has proposed in Japanese Patent Application Publication No.2009-218685 to suppress vibrations of a ribbon by electromagnetic damping when the microphone is not in use.
In the arrangement according to Japanese Patent Application Publication No.2009-218685, there is provided a mechanical switch that turns on (i.e. closed), for example, when a power plug provided on an end of a cable (i.e. cable end plug) on the phantom power supply side is not plugged into an output connector of ribbon microphone and turns off (i.e. open) once the power plug is plugged. The switch is turned on and off across the ribbon.
In this way, when the power plug on the phantom power supply side is not plugged into the output connector and the microphone is not in use, the switch turns on to create an electrically short circuit across the ribbon, resulting in a closed circuit including the ribbon.
In this state, if the ribbon is moved within the parallel magnetic field (i.e. magnetic gap) on impact against the microphone, a back electromotive force is generated in the ribbon. The back electromotive force causes a current to flow through the closed circuit to generate an electromagnetic braking force. Since the braking force acts in the direction opposite to the direction of vibration of the ribbon, the vibrations of the ribbon due to an impact can be suppressed.
As described above, according to the invention set forth in Japanese Patent Application Publication No.2009-218685, when the power plug of the phantom power supply is not plugged into the output connector during transportation or while the microphone is being handled for installation, the movement of the ribbon is restricted by electromagnetic damping even upon impact against the microphone. Thus, the elongation, along with plastic deformation, of the ribbon can be prevented.
If a considerably strong impact force is applied, however, the transverse displacement of the ribbon may not be suppressed to a small level only by the electromagnetic damping.
An object of the invention, therefore, is to ensure that the transverse displacement of the ribbon due to an impact can be suppressed to a level smaller than that achieved by the electromagnetic damping.